It is known to provide an intake system with a supercharger developed with improved charging efficiency by utilizing kinetic effects, such as inertia effects or resonance effects, of intake air by providing two intake pipes, respectively, communicating with two groups of discrete intake passages connected to two groups of cylinders of an engine which in each and the same group are not intended to be fired one after another, respectively. One known type of such an intake system is adapted to increase intake charging efficiency by setting a synchronizing engine speed desirable for providing inertia effects at a low speed. Such an intake system is disclosed in Japanese Patent Publication No. 60-14169 entitled "Intake System For Multi-Cylinder Engine" issued April 11, 1985.
The provision of the intake system described in the above publication, in which inertia effect is intensified in a range of low engine speeds, allows engines with a turbo-charger to compensate for a decrease of intake charging efficiency caused by a decrease of driving force of a turbine of the turbo-charger in a range of low engine speeds.
The use of such an intake system has, however, a drawback that a setting of the synchronizing speed desirable for obtaining favorable inertia effects within a predetermined range of engine speed requires intake pipes whose overall length is considerably long, making it difficult to build a compact intake system.